1. Field of the Art
This invention relates to waveguides for transmitting ultraviolet light, and more particularly to a flexible hollow waveguide for transmitting light emitted from excimer lasers.
2. Prior Art
Ultraviolet light has high photon energy because of the short wavelength. Therefore, by using a photochemical reaction or an ablation effect induced by ultraviolet light, many applications in a variety of fields have been developed. Excimer lasers excited in the wavelength region from 190 nm to 310 nm are especially appropriate for these applications because these lasers emit short light pulses with an extremely high peak power.
In the industrial field, these lasers are utilized in machining process, such as metal cutting and welding, and surface reforming. These lasers are also useful for material synthesis and deposition by using photochemical reaction and laser-assisted CVD method. In the medical field of ophthalmology, dermatology, dentistry and surgery, variety kinds of treatment are performed by utilizing laser ablation of human tissues.
Excimer lasers enable these applications, however, there is a barrier that restrains the laser applications from rapid growth. There exists no definitive delivery system that transmits high-energy light pulses of excimer lasers with high flexibility, low attenuation, and high reliability. Although articulated delivery systems comprising mirrors and arms are used in some applications, the system is not flexible enough for most of applications to deliver the laser beam to the target material. The delivery medium should be highly flexible for easy handling, low cost, and reliable, and also should bear high-energy light pulses of excimer lasers.
The conventional transmitting media of excimer laser beams or another light sources in the wavelengths of ultraviolet region are categorized into two types, silica-based glass fibers and hollow waveguides.
In silica-based glass fibers, both of the core and the cladding are formed by a glass which is mainly composed of fused silica. A common silica-glass fibers which is used in optical communications are not appropriate for delivery of ultraviolet light because impurities such as germanium contained in the glass induces Rayleigh-scattering losses in the short wavelengths. Therefore, some specially designed, silica-based glass fibers have been proposed to transmit the ultraviolet light.
A pure-silica glass fiber is one of the special fibers. Impurities remain in the glass are removed by heat treatment process and the optical fiber made of pure silica show reasonably low attenuation for ultraviolet light. However, the attenuation drastically increases after a number of high-energy pulses of excimer laser are transmitted in the fiber. This is due to E'-center generated in silica by the ultraviolet light or two-photon absorption of silica glass. Because these are intrinsic phenomenon in silica glass, it is hard to remove these effects.
A silica glass fiber doped with fluorine has been also proposed and developed for delivery of ultraviolet light, the energy threshold and reliability are still low because of the above effects of silica glass.
A hollow waveguide is the other type of the transmitting media for ultraviolet light. A hollow waveguide is expected to be the best of delivery media because the core region of the waveguide is the air that shows almost no absorption for ultraviolet light. The following are three types of hollow waveguides which have been proposed and developed, itemized (1), (2) and (3).
(1) A hollow waveguide composed of a metal tube: This type of waveguide is usually composed of a tube that is formed by a metal showing high reflectivity for ultraviolet light, such as aluminum. The inner surface of the metal tube is polished chemically or electrochemically to reduce the scattering loss of light due to surface roughness. By such a polishing, however, it is very hard to form a smooth surface that does not affect the reflectivity of ultraviolet light with the short wavelength. Usually the inner surface roughness of polished metal tube is greater than 500 nm in root-mean-square value that is much larger than the required roughness of 50 nm. Therefore, this type of waveguide has not been successfully developed so far.
(2) A hollow waveguide using a glass tube with a metal coating outside the tube: An example of such a waveguide is disclosed in U.S. Pat. No. 5,276,761. This type of waveguide has an advantage on its easy fabrication process. It is very easy to deposit a metal film on the outside of the glass tube by using a conventional vacuum evaporation or sputtering process. However, the transmission loss of the waveguide is usually high because the reflectivity of the glass material is low even when its outside surface is coated with a metal. Furthermore, a large portion of the energy of transmitted light is confined in the glass wall since a glass is relatively transparent for ultraviolet light and a glass has a higher refractive index than the air core. This causes a fatal damage on the waveguide when a high energy of excimer laser light is launched into the waveguide.
(3) A hollow waveguide with rectangular cross section: The structure and the transmission characteristics of this type of waveguide are reported in "UV laser-biotissue interactions and delivery systemsm," Y. Hashishin, et al., Proc. Soc. Photo-Opt. Instrum. Eng., vol. 2977, pp. 105-114 (1997). As illustrated in FIG. 5, a pair of metal strips 11 and a pair of dielectric spacers 12 form the waveguide with the rectangular core. The metal strips are usually aluminum sheets or phosphor bronze sheets coated with an aluminum film. Because the transmitted light is reflected only at the surface of the metal strips, the material of spacers can be any type of dielectric such as Teflon. The advantage of this type of waveguide is that a polishing of the surface of metal strips is much easier than the polishing of inner surface of the metal tube. The waveguide composed of the aluminum-coated, phosphor bronze strips shows a high transmission and a high-energy threshold for excimer laser light. However, the rectangular hollow waveguide has a disadvantage on the flexibility. The direction of bending is limited by the structure. Furthermore, it is difficult to fabricate a waveguide with a small cross-sectional size. The typical size of the waveguide fabricated so far is a cross section of 8 mm.times.0.5 mm and thus, the waveguide is not flexible enough for applications such as medical treatment.